Along an optical fiber cable (hereafter, simply referred to as optical fiber) set over a great distance, which is used for optical communication and the like, is provided an amplifier, for example, which amplifies light signals.
There is such type of amplifier, which functions based on the principle of amplifying light signals by mixing a pump light with a light signal (for example, Document 1: Ohm-sha, Hideki Ishio, editor, “Light Amplifier and Applications Thereof”, (May 30, 1992) pp. 110-112). This type of amplifier includes a forward pumped-type, reverse pumped-type, and bi-directional pumped-type (Document 1).
FIGS. 7A through 7C are diagrams for describing an optical system for a light amplifier, which utilizes a holding member also called a capillary tube as an optical component that holds the optical fiber. FIG. 7A is a diagram for describing a basic optical system, FIG. 7B is for describing a reverse pumped-type light amplifier, and 7C is for describing a forward pumped-type light amplifier.
With any type, an optical system 10 such as described while referencing FIG. 7A, for example, is applied when inputting a pump light to an optical fiber, which propagates light signals, is necessary. It should be noted that FIG. 7A is a side view for describing this basic optical system 10.
This optical system 10 comprises a first optical fiber 11, which propagates a light signal LS, a second optical fiber 13, which propagates a pump light LP, a holding member 15 as an optical component to which these optical fibers 11 and 13 are inserted and fixed, and a lens 17 and a reflective mirror 19 having filtering characteristics, which are sequentially located in front of the end faces of these optical fibers 11 and 13.
However, the first and the second optical fiber 11 and 13 are inserted in the holding member 15 so as to be mutually parallel. The reflective mirror 19 has characteristics of transmitting the light signal LS and reflecting the pump light LP.
This optical system 10 is utilized, for example, as described while referencing FIG. 7B when it is for a reverse pumped-type light amplifier. Namely, the pump light LP is input to the second optical fiber 13, and is then input to an amplifier 21 via the route from the lens 17, reflective mirror 19, lens 17, and first optical fiber 11. As the amplifier 21, for example, an erbium-doped (namely, doped with erbium) optical fiber amplifier is used.
Furthermore, this optical system 10 is utilized, for example, as described while referencing FIG. 7C when it is for a forward pumped-type light amplifier. Namely, the pump light LP is input to the second optical fiber 13, and is then input to the amplifier 21 via the route from the lens 17, reflective mirror 19, lens 17, and first optical fiber 11.
A conventional example of an optical component configured from such first and second optical fibers 11 and 13 and the holding member 15 is disclosed in Laid-open Japanese Patent Application No. 10-111433, for example. FIG. 8 is a diagram for describing an optical component 30, where the right diagram in the figure is a top view showing an end face of the optical component 30, and the left diagram is a cross-sectional view of the optical component 30 cut along the line X1-X2 of the top view, which is the aforementioned right diagram.
This optical component 30 comprises a short-diameter cylinder 31 as a holding member, a first optical fiber line through-hole 33, which passes through the center axis Q of this short-diameter cylinder 31, and a second optical fiber line through-hole 35, which passes through the short-diameter cylinder 31 parallel to the first optical fiber line through-hole 33 that is eccentric from the center axis, where the hole diameter of the first and the second through-hole 33 and 35 is respectively given as 126 μm.
Furthermore, center axis distance D1 for both through-holes 33 and 35 is given as 0.3 mm (paragraphs 11 and 15 of Laid-open Japanese Patent Application No. 10-111433). Accordingly, both through-holes 33 and 35 are respectively isolated holes.
The aforementioned holding member 31 itself is, as described in paragraph 10 of Laid-open Japanese Patent Application No. 10-111433, is completed as an optical component, which is also called a capillary tube, by casting a ceramic material such as zirconia using a method such as the so-called slip cast method, sintering it, and smoothing the through-hole inner surface through wire polishing or the like.
This capillary tube, as one implementation, for example, is inserted and held in one end of a through-hole of a component, which should be called a capillary tube fixture such as a metallic tube, so as to form an optical component, which is also called a ferrule.
Nevertheless, the holding member 31 that is made of ceramics including zirconia, which is used conventionally, has no transmission for light of wavelengths 0.5 to 2 μm. Accordingly, there is a big problem with the conventional holding member where, for example, in optical communication, in the case where a light signal of a wavelength in the vicinity of 1.5 μm, for example, strikes an end face of the holding member 31, that light signal is reflected diffusely thereat, and at least a portion of that signal becomes a return loss, as is described later.
FIG. 9 is a diagram describing ideal conditions of an optically coupling system between optical fibers and return loss, where the portion representing structural components such as a holding member and a lens is illustrated in a cross section.
In FIG. 9, reference numerals 50 and 60 are holding members, which respectively have at least one through-hole for holding an optical fiber; 50a and 60a are through-holes; 51 and 61 are core portions of the optical fibers; 52 and 62 are cladding portions of the optical fibers; 53 and 63 are metallic members holding the holding members 50 and 60, respectively; 54 and 64 are adhesive layers, which are used according to need; and 55 and 65 are surfaces orthogonal to the center axes of the respective through-holes 50a and 60a (hereafter, also referred to as orthogonal to the respective through-holes) at the end faces of each of the holding members 50 and 60. Reference numerals 57 and 59 are lenses, 58 is a filter, and 67 through 70 are lines indicating optical paths for light signals. Reference numeral 56 is an end face formed by tilting the end face 55, which is orthogonal to the through-hole 50a, 8 degrees clock-wise in the figure, and 66 is an end face formed by tilting the end face 65, which is orthogonal to the through-hole 60a, 8 degrees counter clock-wise in the figure. It should be noted that with FIG. 9, for convenience of explanation, the scale size of each part such as the holding members 50 and 60, the metallic members 53 and 63, the core portions 51 and 61, the cladding portions 52 and 62, the adhesive layers 54 and 64, the lenses 57 and 59, and the filter 58 are not the same, and also for convenience of explanation, they are illustrated with differing scales according to need.
With FIG. 9, light having progressed through the core portion 51 from left to right of the figure goes out to space from the end face 55 of the core portion, spreads into an optical beam as indicated by reference numeral 67 and passes through the lens 57, becoming parallel optical beams as indicated by reference numeral 68. It then passes through the filter 58 and becomes an optical beam as indicated by reference numeral 69, passes through the lens 59 becoming an optical beam as indicated by reference numeral 70, and finally enters the core portion 61.
However, the optical beam illustrated in FIG. 9 is an ideal one, where with actual optical coupling, the optical beam spreads more than illustrated in the figure, and all light having progressed towards the core portion 61 does not necessarily enter therein.
In practice, without all light progressing towards the core portion 61 entering the core portion 61, a portion thereof is reflected diffusely due to the cladding portion 62 or the holding member 60 or the like, where a portion thereof progresses reversely in the order of the lens 59, the filter 58, and the lens 57. This results in loss that is called a return loss.
With such an optically coupling system, minimizing this return loss is a key issue. A method that is implemented conventionally for that reason forms the end face 56 tilted 8 degrees by inserting and fixing in the through-hole 50a an optical fiber configured from the core portion 51 and the cladding portion 52 so that the end face 55 of the holding member 50 on the light emitting side can become the end face tilted 8 degrees as aforementioned, and then polishing the end face of the holding member 50 and the end face of the optical fiber configured from the core portion 51 and the cladding portion 52. Doing so may prevent adverse effects from the return light. Moreover, prevention of adverse effects from the return light is attempted by making the plane of incidence and the output plane of the filter 58 also tilt, as well as making the end face of the holding member 60, which holds the core portion 61 and the cladding portion 62 on the light receiving side, tilt 8 degrees as aforementioned into the end face 66.
Nevertheless, the adverse effects from the return light have not been resolved. Namely, as aforementioned, all of the light emitted from the optical fiber on the light emitting side does not have to be incident on the core portion 61 of the optical fiber on the light receiving side at the end face of the holding member 60, which configures the optical fiber terminal portion on the light receiving side, but a part of the light may be incident on the cladding portion 62 or the like as illustrated in FIG. 10.
FIG. 10 is a cross-sectional view for describing light that has entered a cladding portion.
In FIG. 10, reference numerals 71, 71a, 72, and 72a are lines respectively indicating light rays, and A1 and A2 are points indicating the positions thereof.
In FIG. 10, light progressing from left to right in the figure as indicated by reference numerals 71 and 72 enters the cladding portion 62 at the end face 66 of the holding member 60 on the receiving side and reflected at the core portion 61, progresses as indicated by the respective reference numerals 71a and 72a, and then reaches the periphery of the cladding portion 62 as indicated by A1 and A2, respectively. The cladding portion 62 is fixed with adhesive in the through-hole 60a. The inner surface of the through-hole 60a is a polished surface. The light indicated by the reference numerals 71a and 72a are reflected near the positions indicated by A1 and A2, respectively, of which a part becomes the aforementioned return loss.
This light that becomes a return loss partially returns to the light source, for example, and greatly disrupts the light-emitting condition of a laser diode as the light source; and a portion thereof provides adverse effects to light signals.
Presently in optical communication, it is important that the return loss due to this reflected light is small.
Another exemplary method attempting to reduce this return loss where the holding members 50 and 60 (also referred to as a capillary tube) are made of glass other than the conventional mainstream zirconia ceramics is proposed.
However, since a glass holding member cannot undergo hole processing and finishing through wire polishing (for example, cannot be given a glaze), finishing a hole with very close dimension tolerances as with a capillary tube is impossible, whereby it was manufactured by a method of stretching a thick glass tube with an outer diameter and a hole diameter analogous to those of a capillary tube. Accordingly, the glass holding member had large drawbacks when used for an optical fiber terminal where accurateness of optical coupling was poor due to poor accuracy in positioning the core portion, as well as being difficult to process.
Furthermore, in the case of using the glass holding member, for example, when attaching the glass holding member to the metallic member 63 of FIG. 9 or FIG. 10, the glass easily breaks, attaching through driving in (press fitting) is impossible, and fixing with an adhesive or the like is necessary, where in addition to poor accuracy in positioning each portion as compared to the case of attaching by driving in, light scatters due to the adhesive. Therefore, as aforementioned, the light that is not incident on the core portion 61 but is incident on the cladding portion 62 or the glass parts of the holding member 60 has drawbacks of being dispersed when reaching the boundary of the metallic member 63 by a scatterer such as an adhesive layer 64 that exists there, leading to an increase in return loss.
The present invention resolves such problems and provides an optical component as a holding member for holding an optical fiber terminal portion, which has small return loss, can be easily processed and handled, is suitable for mass production, and provides low manufacturing costs when used for optical communication by holding an optical fiber.